The field of thin film ceramics has seen rapid growth in recent years. This activity stems in large part from the attractive and often unique properties that ceramic materials bring to a wide variety of applications, including: thin film ferroelectrics; magnetic recording; multilayer coatings for lenses, windows, and laser optics; hard, corrosion-resistant coatings for optical fibers; filters for electromagnetic radiation; acousto-optic devices; and electrochemical sensors for detection of combustible or hazardous species in gases. Another essential ingredient in these developments has been the growing progress in thin film deposition technology, involving gas phase techniques such as chemical vapor deposition, sputtering, laser ablation, and evaporation.
Despite their successes, these deposition techniques have some significant shortcomings. Capital equipment costs can be prohibitively high, especially for large-volume applications. There is considerable art associated with the design of the deposition systems and with the control of the operating parameters (e.g., to correct for differential deposition rates between components in a multicomponent film). The most common techniques still primarily involve line-of-sight deposition, with limited applicability to particulates and to complex surfaces and shapes. Most importantly, achievement of the desired properties often requires that the substrate be heated to significant temperatures (several hundreds of .degree.C.), either during deposition or subsequently, to convert the usually amorphous as-deposited material into a well-ordered crystalline film. This significantly limits the capabilities of thin film deposition technology for many metal and polymeric substrates. When a specific crystallographic orientation of the thin film is desired, the choice of substrate is usually further restricted to a single-crystal material whose surface interatomic spacing closely matches that of the desired crystalline film, enabling epitaxial growth.
Liquid phase thin film preparation techniques are also known in the art and involve homogeneous precipitation of colloidal particles or gel solutions which are cast as films onto substrates. These methods include sol-gel, colloidal particle systems, spray pyrolysis, and electroless and electrodeposition. Obstacles are associated with these techniques with respect to thin film formation. Colloidal systems, for example, can have low densities and uncontrolled microstructures as a result of the use of binder phases or the aggregation of colloidal particles. Polymeric sol-gel solutions can generate amorphous and porous structures which manifest in low density films. Problems with film shrinkage and cracking are exhibited during scale up of sol gel techniques. Electroless deposition techniques are essentially limited to metal deposition and call for the use of catalysts for metal reduction and provision of nucleation sites on substrate surfaces. Both gas phase and liquid phase thin film formation techniques have limitations. Gas phase deposition generally calls for expensive equipment and line of sight deposition. Liquid phase deposition can result in cracking and shrinkage. Other limitations and disadvantages exist with respect to known thin film formation techniques.
With the above in mind, numerous instances of inorganic material synthesis by living organisms stand out as striking suggestions for alternate synthetic deposition routes. The microstructures of many shells, teeth, and other biological inorganic structures are remarkably dense and uniform. They often exhibit precise crystallographic relationships maintained over millimeter or centimeter length scales. Mechanical properties in many instances exceed those of identical compounds fabricated via manmade or geological routes. And in all known instances, the materials are synthesized at or near room temperature from aqueous solutions. Furthermore, growth of the inorganic phase usually occurs in the presence of a thin organic substrate, which dictates not only the location of the reaction but the crystallographic orientation and eventual macroscopic shape of the structure.
The present invention is directed to the deposition of metal-containing oxide films onto an organic template commonly referred to as a self-assembled monolayer (SAM). The resulting thin film is a uniform, densely packed structure finding usefulness in a wide variety of areas.